Acoustic feedback is a common problem encountered in any typical closed loop audio amplification system. Generally speaking, feedback arises in an amplification system at frequencies where the loop gain of the electronic circuit is greater then or equal to unity and the system phase is positively reinforced such that particular frequencies may grow uncontrollably. In such a system, feedback will occur at the specific frequencies that satisfy these specific gain and phase criteria. The magnitude of the feedback frequencies will grow at a rate that is determined by the gain at those frequencies and a delay associated with various parameters of the amplification system. Once feedback instability is initiated, typically the only way to stop the growth of the magnitudes of signals at the feedback frequencies is to reduce the overall system gain below the point of unity gain. Then, the growing feedback frequencies will decay and go away, but at the expense of an overall reduction in gain across the entire frequency spectrum of the amplification system.
In the past, a number of feedback detection and feedback correction/suppression systems have been employed in an effort to deal with feedback. Identification and control of feedback may be most pertinent to a live sound reinforcement setting where a sound engineer must quickly be able to deal with feedback by identifying the fact that feedback exists and determining the best and fastest way to deal with the feedback (i.e., eliminate it or at least reduce it) without dramatically affecting the overall signal, (i.e., the sound presentation of the performance).
Several past solutions regarding feedback problems involve identifying sustained magnitudes of signals at a set frequency and then reducing overall gain or applying a notch filter at the identified frequency. In one example, Patronis, U.S. Pat. No. 4,079,199 (granted Mar. 14, 1978), discloses a system that detects a persistent frequency component, then reduces the broad band system gain until the persistent frequency component goes away. Such a system, though successful at stopping feedback, has drawbacks. The purpose of the audio amplification system is to increase the gain of the person speaking/playing into the microphone so that he or she can be heard. If the feedback suppressor reduces the overall gain, then the effectiveness of the amplification system is negated because the overall signal gain is reduced by the feedback suppressor at an equivalent rate in which the amplifier may attempt to increase the overall signal gain. Furthermore, some acoustic sound waves having persistent frequencies, such as a holding a long note or providing a solid steady bass tone, are typically part of musical performances. A system that employs such a feedback suppressor will often mistakenly turn down the amplification of musical instruments when sustained musical notes are present.
In an attempt to make the suppression of feedback less noticeable, Chen, U.S. Pat. No. 4,091,236 (granted May 23, 1978) discloses using automatically tunable notch filters to narrow the frequency band that is affected by a gain reduction. Thus, instead of reducing all frequencies across the audio spectrum, only frequencies in which a sustained presence of signal magnitude is detected (which is identified by prior art systems as feedback, regardless of whether or not it actually is) are reduced. Of course, this solution still does not address distinguishing between actual feedback and sustained musical tones.
Later developments in the prior art have begun attempting to distinguish between actual feedback and sustained musical tones. In specific, Lewis et al., U.S. Pat. No. 5,245,665 (granted Sep. 14, 1993) disclose a system that is capable of some discrimination of musical tones from feedback. In this disclosure, the system attempts to distinguish musical tones by identifying harmonics that are typically produced by most musical instruments. That is, almost all instruments (including the human voice) produce notes at various fundamental frequencies in which the human ear can easily identify (e.g. middle C, A#, etc.). These instruments also produce harmonics that allow the human ear to distinguish between instruments (e.g., a flute produces harmonics different form a female voice which is, in turn, different form a male voice). Using this natural phenomena, the harmonic content of the input signal may be analyzed by the feedback suppressor system of Lewis et al. to distinguish between fundamental frequencies that have related harmonics (probably musical notes) and fundamental frequencies that do not have related harmonics (probably feedback). If the feedback analyzer finds a fundamental frequency being greater then any of its harmonics or sub harmonics by a predetermined factor (such as 33 dB), then the identified frequency may be designated a feedback component which triggers the deployment of a notch filter.
While checking for harmonics does give an added measure of protection against the false identification of music as feedback, there are still shortcomings with this method. Multiple feedback frequencies can arise naturally that are harmonically related to not only the feedback itself but possibly to other actual musical notes. Natural distortion of the audio amplification system can also produce harmonics that are similar in strength to those generated by musical instruments. When these situations arise, the prior art fails to place notch filters allowing feedback to go unchecked. Conversely there are times when desirable sinusoidal signals with weak harmonics are detected falsely by the system as feedback and notches are deployed causing unnecessary degradation of the system. These signals include test tones commonly used to test the audio amplification system, a human whistle and other more tonal instruments such as a flute.
As a result, analyzing a signal for feedback based on related harmonics of sustained frequencies is wrought with potential misapplication of feedback suppression filters. The prior art cannot be relied upon to effectively distinguish sustained musical tones from feedback, especially as musical instruments gravitate from acoustic to electronic where harmonics are less prevalent.